In magnetic resonance imaging, the body of a subject is positioned in a primary field magnet and subjected to a strong, constant magnetic field. Radio frequency signals are applied to the subject, which causes the spin axes of certain atomic nuclei within the body of the subject to precess or rotate around axes parallel to the direction of the magnetic field. The precessing nuclei emit weak radio frequency signals referred to herein as magnetic resonance signals.
By applying small magnetic field gradients to the subject along with the static magnetic field, the magnetic resonance signals can be spatially encoded so that it is possible to recover information about individual volume elements or “voxels” within the subject's body from the magnetic resonance signals. The information can be used to reconstruct an image of the internal structures within the body. Because magnetic resonance imaging does not use ionizing radiation, it is inherently safe. Moreover, magnetic resonance imaging can provide excellent images depicting structures which are difficult to image using other modalities.
The quality of a magnetic resonance image depends strongly on the quality of the static magnetic field. To provide an optimum image, the static magnetic field must be both strong (typically on the order of 0.5 Tesla or more) and uniform to about 1 part in a million or better, desirably about 1 part in 107 or better. Some magnetic resonance imaging instruments employ air core superconducting static field magnets. These magnets typically have electromagnet coils formed from superconducting materials arrayed along an axis so that the coils cooperatively form an elongated solenoid surrounding the axis. The coils are cooled to cryogenic temperatures, typically about 4.2° Kelvin (approximately −267° C. or −450° F.). At these temperatures, the coils have no electrical resistance. The superconducting coils can conduct large currents and provide a strong magnetic field.
Magnets of this type typically have a housing defining an elongated bore extending along the axis and require that the patient enter into this bore. The bore may be about 1 meter in diameter. Thus, the patient is subjected to a highly claustrophobic experience during imaging, akin to lying on a stretcher inside a drain pipe. Moreover, these magnets cannot be used to image patients who are extremely obese, or who require bulky life support equipment during imaging. Additionally, air core magnets have strong fringe fields extending outside of the magnet housing. These fields can attract ferromagnetic objects in the vicinity of the magnets with such strength that the objects turn into deadly missiles. Despite stringent precautions taken by imaging centers to prevent entry of ferromagnetic objects into the danger zone surrounding a magnet, accidents have occurred resulting in injuries and deaths.
Iron core magnets use a ferromagnetic frame defining a flux path and usually include ferromagnetic poles projecting towards a patient-receiving space from opposite sides, so that the pole tips define the patient-receiving space between them. The ferromagnetic frame effectively eliminates the fringe field outside of the frame. Moreover, the ferromagnetic frame serves to concentrate the field within the patient-receiving space and provides a low-reluctance flux path. Ferromagnetic frame magnets can provide the requisite field strength using essentially any source of magnetic flux, including superconducting coils, resistive coils, or masses of permanent magnet material.
One particularly desirably ferromagnetic frame magnet is disclosed in commonly owned U.S. Pat. No. 6,677,753, the disclosure of which is hereby incorporated by reference herein. As disclosed in preferred embodiments of the '753 patent, the frame includes ferromagnetic side walls extending generally vertically and flux return structures extending generally horizontally above and below the patient-receiving space. Poles project from the side walls toward the patient-receiving space. As described in greater detail in the '753 patent, a patient may be positioned within the patient-receiving space in essentially any orientation relative to gravity, and may be moved relative to the frame so as to position essentially any part of the patient's body within the patient-receiving space, in the vicinity of the magnet axis extending between the poles.
Preferred magnets according to this general structure can provide extraordinary imaging versatility. For example, a patient may be imaged lying in a recumbent, substantially horizontal position and then imaged again while in a substantially vertical position such as standing or sitting. Comparison of these images can yield significant information about certain conditions. Also, these magnets provide an open environment for the patient.
As mentioned above, a ferromagnetic frame magnet can use any source of magnetic flux, including superconducting coils. A magnet using superconducting coils in conjunction with the ferromagnetic frame can provide very high field strength with good uniformity. U.S. Pat. Nos. 6,323,749 and 4,766,378, for example, disclose certain arrangements for mounting superconducting coils on ferromagnetic frame magnets.
Despite all of this progress in the art, still further improvement would be desirable. As mentioned above, superconducting coils must be maintained at cryogenic temperatures, typically at the temperature of liquid helium (about 4.2° Kelvin or below). Typically, the coils are provided with refrigeration units referred to herein as cryocoolers, which can abstract heat from the coil and from associated components even at this very low temperature. However, the cryocoolers have only a very limited capacity, typically on the order of a few watts or less at this temperature. Therefore, the coils must be surrounded by very efficient thermal insulation. Most commonly, the coils are enclosed in vessels which are maintained under hard vacuum. The coils must be supported and held in place within the vessels. The structures hold the coil must resist not only the weight of the coil but also the magnetic forces generated during operation. Depending on the particular coil design and the design of any adjacent ferromagnetic frame elements, these forces can be on the order of tons. The coil vessel and supporting structure should be compact so as to minimize the size and weight of the apparatus. Design of a compact coil enclosure and supporting system has presented a challenge heretofore.
Cryocoolers typically induce mechanical vibrations. Transmission of such vibrations to the coils and associated structures tends to degrade the uniformity and stability of the static magnetic field. However, it is generally desirable to position portions of the cryocooler in proximity to the coil enclosure. This further complicates design of such a system. Additionally, the superconducting coils used heretofore typically have not been arranged for optimum co-action with the ferromagnetic frame. Thus, further improvement would be desirable.